Tuesday, July 15, 2014

Birt E and the fiery rift valleys of Nubium

LROC Narrow Angle Camera-derived Digital Terrain Model (DTM, color gradations superimposed) on a close-up view along the northeastern long axis of Birt E (5.34 km; 20.72°S, 350.337°E), the vent structure of Rima Birt, a sinuous rille in east Mare Nubium immediately west Rupes Recta. The vent is thought to be a source region for pyroclastic flows that traced out Rima Birt. The mare basalt, visible as slightly darker material in a fan spreading north from the vent, is older than 3.4 billion years. A roughly 3 km-wide field of view from LROC NAC mosaic M1144849711LR, LRO orbit 20750, January 20, 2014; 46.21° incidence angle, 84 cm resolution from 78.77 km over 20.69°S, 351.11°E [NASA/GSFC/Arizona State University].

Aaron Boyd

LROC News System

Birt E (20.72°S, 350.337°E) was not created like most craters on the Moon; there was no meteorite impact. Lava sputtered out of this pyroclastic vent in Mare Nubium over 3.4 billion years ago, dispersing lava onto the surface and leaving the crater we see today.

How can we tell it is a volcanic vent and not an impact crater?

Impact craters and volcanic vents can be differentiated because vents often have an irregular or elongated shape (as with Birt E). Impact craters are usually circular in shape, created by the shockwave during an impact event.

Pyroclastic vent structure at Rima Birt, officially "Birt E," though it is not a crater; west of Rupes Recta in east Mare Nubium. 4.66 km-wide field of view from LROC NAC mosaic M1144849711LR [NASA/GSFC/Arizona State University].

Also, the vee-shape of this crater is likely a product of the formation mechanism. Vee-shaped vents are thought to be formed from a pyroclastic eruption. Gasses fractionating out of the liquid rock create violent events during eruptions. Explosive eruptions created the shape that we see today, but Birt E could have had a complex history with effusive eruptions forming Rima Birt, the short sinuous rille flowing from Birt E to the south-southeast.

East Mare Nubium, an orbital view north from 100 km altitude, an HDTV still from Japan's lunar orbiter Kaguya (SELENE-1) in 2008. (See the wallpaper-sized release HERE.) [JAXA/NHK/SELENE].

Over long enough time scales Birt E will be filled in with ejecta from newly formed craters around Mare Nubium or by mass wasting of the walls into the crater. Let’s enjoy this ancient crater today while we still can!

As the GRAIL gravity probes mapped the Moon's basic anisotropy in 2012 detecting the densities of "deep fiery rift valleys," a probable source of a half billion year period of pyroclastic volcanism surrounding the Moon's Procellarum terrain, added important pieces to the tossed puzzle of the Moon's morphology. In the newest maps of the Moon's own squarish "ring of fire," there is still a missing link under the Southern Highlands, but a source for the multiple inundations of the nearside's lowlands, weight that may have created the Straight Wall fault in east Mare Nubium, may have been found [NASA/GSFC/SVS].

LOLA laser altimetry color-coded over LROC WAC 100 meter global mosaic demonstrates where Rupes Recta marks a neat break in the continuity of the floor of an otherwise almost entirely erased 190 km crater. The weight of repeated inundations gradually sloped that crater's floor 1500 meters in elevation deeper on the west side, gradually, aside from the 100 to 300 meter "snap" represented by Rupes Recta [NASA/GSFC/Arizona State University].